**8. Duplex ultrasound assessment of carotid plaque morphology**

Duplex ultrasound is arguably the most important imaging modality for preoperative assessment of patients with carotid atherosclerotic disease. It is non-invasive, relatively inexpensive and very accurate at identification of significant ICA stenosis. [87-92] In measuring the degree of stenosis, the flow and velocity characteristics assessed by colour flow Doppler are utilized. Duplex devices also generate high resolution B-mode ultrasound images of the atherosclerotic lesion. These images do not contribute significantly to the assessment of carotid artery stenosis. However the B-mode ultrasound image can be used to assess morphologic characteristics of an atherosclerotic lesion. It has been known for some time that plaques that have low echogenicity (appear dark on Duplex ultrasound) or a high degree of heterogeneity are associated with histologic characteristics of plaque instability, ipsilateral neurological or ocular events, [93] CT evidence of carotid territory cerebral infarction or evidence of emboli‐ sation on trans-cranial ultrasound. [94] These ultrasound characteristics can be assessed subjectively and classified by a trained observer.

The use of stratified GS median measurements which create a profile of the regional GS median as a function of distance from plaque surface combined with colour mapping correlates better with the presence of various histopathological components and identify determinants of

Carotid Plaque Morphology: Plaque Instability and Correlation with Development of Ischaemic Neurological Events

http://dx.doi.org/10.5772/57254

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**Figure 4.** Calculation of Grey Scale Median of a hypo-echoic plaque. (*Swiss Med* 2005; 135:635–643.)

**9. Magnetic resonance imaging assessment of plaque morphology**

Magnetic resonance imaging (MRI) is a promising modality for characterisation of carotid plaque morphology and assessment of composition of atherosclerotic plaques. It can accu‐ rately identify the presence of ulcerated or thin plaque cap, [105-107] quantify intra-plaque haemorrhage [105-107], or the presence of a large necrotic plaque core [105-107]. Serial MRI

plaque instability with a high degree of accuracy. [94]

B-mode ultrasound assessment of atherosclerotic plaque morphology started some 30 years ago. Reilly *et al* recognised two distinct types of carotid atherosclerotic lesion. The first was termed homogenous and was defined as lesion with uniformly high or medium level echoes. Histologically, homogenous plaques are fibrous lesions. [95] The second type was termed heterogeneous and was defined as plaque with high, medium and low level echoes. [95]

Histologically heterogeneous plaques contain variable amounts of intra-plaque haemorrhage, lipids, cholesterol crystal and a loose stroma. A further refinement of subjective assessment of plaque morphology was the Gray-Wheal classification method (table-3). [96] In the Cardio‐ vascular Health Study, which enrolled 5,201 individuals aged 65 years and over without prior cerebrovascular symptoms, and followed them for an average of 3.3 years demonstrated a significantly increased incidence of stroke in individuals who had echo-lucent plaques. [97]


**Table 3.** Gray-Wheal Classification of atherosclerotic plaques

Subjective observer dependent assessment of plaque morphology, whilst useful, is limited by high inter- and intra-observer variability, significantly limiting its clinical application. [98-99] Echogenicity and heterogeneity of an atherosclerotic plaque can be objectively assessed using image analysis techniques through the measurement of median grey scale (GS) value of the ultrasound image, percentage of echo-lucent pixels and entropy in GS characteristics of the lesion(figure-4). [99-103] In order to remove variability associated with acquisition of the ultrasound image, the US images are normalised using linear scaling so that the adventitia would have a grey scale median value of 185-195 and blood 0-5. Plaques with a low GS median were associated with a significantly higher annual risk of stroke. [99-104]

Interestingly, although characterisation of the internal structure of the plaque assessed by image analysis correlates closely with clinical symptoms, the correlation between computer‐ ised assessment of plaque morphology and histological features of the lesion is less strong. [94] This indicates that values such as GS median represent a median value of the whole athero‐ sclerotic area and do not necessarily reflect the presence of particular regional components. The use of stratified GS median measurements which create a profile of the regional GS median as a function of distance from plaque surface combined with colour mapping correlates better with the presence of various histopathological components and identify determinants of plaque instability with a high degree of accuracy. [94]

artery stenosis. However the B-mode ultrasound image can be used to assess morphologic characteristics of an atherosclerotic lesion. It has been known for some time that plaques that have low echogenicity (appear dark on Duplex ultrasound) or a high degree of heterogeneity are associated with histologic characteristics of plaque instability, ipsilateral neurological or ocular events, [93] CT evidence of carotid territory cerebral infarction or evidence of emboli‐ sation on trans-cranial ultrasound. [94] These ultrasound characteristics can be assessed

B-mode ultrasound assessment of atherosclerotic plaque morphology started some 30 years ago. Reilly *et al* recognised two distinct types of carotid atherosclerotic lesion. The first was termed homogenous and was defined as lesion with uniformly high or medium level echoes. Histologically, homogenous plaques are fibrous lesions. [95] The second type was termed heterogeneous and was defined as plaque with high, medium and low level echoes. [95]

Histologically heterogeneous plaques contain variable amounts of intra-plaque haemorrhage, lipids, cholesterol crystal and a loose stroma. A further refinement of subjective assessment of plaque morphology was the Gray-Wheal classification method (table-3). [96] In the Cardio‐ vascular Health Study, which enrolled 5,201 individuals aged 65 years and over without prior cerebrovascular symptoms, and followed them for an average of 3.3 years demonstrated a significantly increased incidence of stroke in individuals who had echo-lucent plaques. [97]

Subjective observer dependent assessment of plaque morphology, whilst useful, is limited by high inter- and intra-observer variability, significantly limiting its clinical application. [98-99] Echogenicity and heterogeneity of an atherosclerotic plaque can be objectively assessed using image analysis techniques through the measurement of median grey scale (GS) value of the ultrasound image, percentage of echo-lucent pixels and entropy in GS characteristics of the lesion(figure-4). [99-103] In order to remove variability associated with acquisition of the ultrasound image, the US images are normalised using linear scaling so that the adventitia would have a grey scale median value of 185-195 and blood 0-5. Plaques with a low GS median

Interestingly, although characterisation of the internal structure of the plaque assessed by image analysis correlates closely with clinical symptoms, the correlation between computer‐ ised assessment of plaque morphology and histological features of the lesion is less strong. [94] This indicates that values such as GS median represent a median value of the whole athero‐ sclerotic area and do not necessarily reflect the presence of particular regional components.

subjectively and classified by a trained observer.

92 Carotid Artery Disease - From Bench to Bedside and Beyond

Plaque Type Ultrasound characteristics

Type-1 Predominantly echolucent with a thin echogenic cap

Type-2 Intermediate echolucent lesions with small areas of echogenicity

Type-4 Uniformly echogenic lesions (equivalent to homogenous).

**Table 3.** Gray-Wheal Classification of atherosclerotic plaques

Type-3 Intermediate echogenic lesions with small areas of echolucency (<25%)

were associated with a significantly higher annual risk of stroke. [99-104]

**Figure 4.** Calculation of Grey Scale Median of a hypo-echoic plaque. (*Swiss Med* 2005; 135:635–643.)
